<!DOCTYPE html>
<html>

	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>

	<body>
		<style type="text/css">
			* {
				margin: 0;
				padding: 0;
			}
			
			.link {
				width: 300px;
				margin: 0 auto;
				border: 1px;
			}
			
			.link ul {
				clear: both;
				
			}
			
			.link ul li {
				list-style: none;
				float: left;
				padding: 20px;
				text-align: center;
				position: relative;
			}
			
			.link ul li a {
				color: #000000;
				text-decoration: none;
			}
			
			.link ul li a:hover {
				color: lightpink;
			}
			
			
			.my_link {				
				width: 0px;
				height: 2px;
				background-color: pink;
				bottom: 20%;
				left: 50%;
				transform:translateX(-50%);				
				position: absolute;
				transition-property: all;  
				/*属性*/
				transition-duration: 1s;
				/*持续时间*/
				transition-timing-function:ease-in-out;
				/*速度线性*/
				transition-delay: 0s;
			}
			.link ul li a:hover+.my_link{				
				width: 48px;																		
			}
		</style>
		<div class="link">
			<ul>
				<li>
					<a href="#">我是一</a>
					<div class="my_link"></div>
				</li>
				<li>
					<a href="#">我是二</a>
					<div class="my_link"></div>
				</li>
				<li>
					<a href="#">我是三</a>
					<div class="my_link"></div>
				</li>
			</ul>
		</div>
	</body>

</html>